The use of composite materials formed by an organic matrix and unidirectionally orientated fibres, such as Carbon Fibre Reinforced Plastic (CFRP), in the manufacture of structural components of an aircraft, for example fuselage skin panels, torsion boxes, stringers, ribs, spars etc., is well known in the aeronautical industry.
Typically, skin panels are stiffened by several stringers longitudinally arranged, in order to provide strength and guarantee a proper buckling behavior of the skin panels. The stringers are conventionally co-cured, co-bonded, secondarily bonded or bolted to the skin panel. FIG. 1 shows a conventional design of a stringer (3) bonded to a skin panel (1).
At the ends of the stringer (3), the stringer load is transferred to the skin panel (1), so that a redistribution of loads is originated at the stringer terminations, which causes well known de-bonding problems (due to peeling and shear effects) between the stringer (3) and the skin panel (1). In order to reduce the stress concentration at the stringer termination and mitigate the associated problems, stiffeners are conventionally manufactured with the so called stringer “run-out” section (5) at the ends which contributes to improve the load transfer from the stringer (3) into the panel (1) reducing the stress concentration at the end of the stringer (3).
At the run-out section (5), the cross-section of the stringer (3) is progressively reduced towards the end by reducing the height and/or thickness of the stringer (3), so as to progressively reduce the load supported by the stringer (3) at the run-out (5). Therefore, a conventional stringer (3) design has a tapered termination (6).
U.S. Pat. No. 4,606,961, and US patent applications US-2005/0211846 and US-2012/0100343, are examples of these techniques.
In addition to the use of stringers with tapered web at the run-out, other approaches (to guarantee proper load transference at this structural detail) are known, which are based on the use of additional components such as metallic brackets or bolts. U.S patent application US-2012/0234978A1, and PCT publication WO2012/042246A2, are examples of that type of solution.
Despite the fact that the above-mentioned solutions satisfactorily enhance the strength of the union between stringer and panel, those solutions are affected by the drawback that the manufacturing process becomes more complex and in most of the cases, more expensive, since it is necessary to manufacture an additional component (the metallic insert or bracket), which has to be subsequently fitted, for example bolted, to the skin panel.
Due to the before-mentioned problem and taking into account the need for structural solutions that provide integration, the need in the manufacture of composite structures for improved solutions which, assuring the strength of the union between stringer and panel to avoid de-bonding problems, do not require the provision of additional components or substantial modifications of the manufacturing process has been detected.